In the traditional transformation of metals into finished products through the use of cavity moulds, for instance by casting, forging, stamping or sintering, the dimensional precision often is insufficient. The practice thereof requires expensive installations and high professional qualifications, which renders manufacture in large series attainable only with difficulty.
In powder metallurgy, the finely divided metal is moulded under high pressure and then subjected to thermal sintering. By sintering, here one understands an operation carried out in powder metallurgy so as to bring about an agglomeration of the treated products in order to give them sufficient cohesion and rigidity. When one heats to a high temperature grains of metallic powder contacting one another by compression, there is brought about a true welding of the grains before their melting temperature has been attained, and one obtains a rigid and compact metallic mass. However, at the time of removal from the mould and prior to sintering, the parts are very sensitive to manipulation in view of their lack of cohesion. Additionally, sintering is accompanied by a relatively substantial shrinkage which requires corrections to the mould. Small parts and those exhibiting complex forms then pose multiple manufacturing problems and such procedure has definite limits.
In the horological industry, it has often been proposed to form watch cases by the sintering technique. An example of a case thus obtained is described in the patent document CH-A-538 721 where the shrinkage problem is mentioned.
The patent document US-A-4 194 040 claims a semi-finished material composed of metallic powder into which is mixed 85 to 99% of polytetrafluorethyline resin fibrils. This material as such exhibits no mechanical strength and it will thus be necessary to sinter it at high temperature in order to bestow thereon a good strength. During the sintering, the resin forming the bond among powder grains disappears by pyrolysis and the finished material is 100% metallic.
The patent document US-A-4 595 558 also claims a semi-finished material in the form of briquettes composed of a mixture of manganese and aluminium powder. To this mixture is added a 0.5 portion in volume of polyglycol. Such briquettes are then soaked in molten aluminium, this having as result a finished product exhibiting a manganese aluminium alloy, the polyglycol having totally disappeared.
Furthermore, there are known thermoplastic materials having a metallic powder filler and moulded by injection. Such thermoplastic materials require a melting viscosity adapted to completely fill the mould. The proportion of metal may hardly exceed about 40% in volume. This has as a consequence that the characteristic properties of the metals are diminished and the characteristic properties of the thermoplastics become dominant.
There is also known transfer moulding of a mixture of thermosetting material with finely divided metal powder. But here the flow of the molten material is of short distance. It is however possible to mould mixtures having a higher molten viscosity which enables the proposal of a higher proportion of metal, but still hardly exceeding 50% by volume. It will also be noted that the hardening of the mixture by polyaddition or polycondensation must take place in the mould itself which prolongs considerably the manufacturing cycles and leads to low productivity. Here also, the characteristic properties of the thermosetting materials are those which predominate to the detriment of the characteristic properties of the metals.